There is an increasing requirement to be able to produce microcomponents to be integrated onto supports other than those used to fabricate them.
Examples that may be cited are microcomponents on plastics material or flexible substrates. Here the term “microcomponent” means any wholly or partially processed electronic or optoelectronic device or any sensor (for example any chemical, mechanical, thermal, biological or biochemical sensor).
A layer transfer method can be used to integrate such microcomponents onto flexible supports.
There are many other examples of applications in which layer transfer techniques may be suitable for integrating microcomponents or layers onto a support that is unsuitable for their fabrication a priori. In the same line of thinking, these layer transfer techniques are also very useful when it is required to isolate a thin layer, with or without microcomponents, from its original substrate, for example by separating or eliminating the latter. Also in the same line of thinking, turning over a thin layer combined with transferring it onto another support provides engineers with a valuable degree of freedom to design structures that would otherwise be impossible. Thin films can be separated and turned over for example to produce buried structures such as buried capacitors for dynamic random access memories (DRAM) in which, unusually, the capacitors are first formed and then transferred onto another silicon substrate before fabricating the remainder of the circuits on this new substrate. Another example concerns the processing of dual gate transistor structures. The first gate of a CMOS transistor is produced on a substrate using a conventional technology and then transferred and turned over onto a second substrate to produce the second gate and finish the transistor; the first gate is therefore buried in the structure (see for example K. Suzuki, T. Tanaka, Y. Tosaka, H. Horie and T. Sugii, “High-Speed and Low-Power n+-p+ Double-Gate SOI CMOS”, IEICE Trans. Electron., vol. E78-C, 1995, pages 360 to 367).
The requirement to isolate a thin layer from its original substrate is encountered in the field of light-emitting diodes (LED), for example, as reported in the following documents, for example: W. S. Wong et al., Journal of Electronic Materials, page 1409, vol. 28, No. 12, 1999, and 1. Pollentier et al., page 1056, SPIE vol. 1361, “Physical Concepts of Materials for Novel Optoelectronic Device Applications I”, 1990. Here one of the aims is improved control of extraction of the light emitted. Another aim relates to the fact that in this particular example the sapphire substrate used for the epitaxial stack is bulky a posteriori, in particular because of its electrically insulative nature, and this prevents making electrical contact to its rear face. It therefore seems now to be desirable to be able to dispense with this sapphire substrate, the use of which was advantageous in the phase of growing the material.
An identical situation is encountered in the field of applications associated with telecommunications and microwaves, for example. In this case, it is preferable for the microcomponents to be finally integrated onto a support having a high resistivity, typically at least several kohm·cm. However, a highly resistive substrate is not necessarily available at the same cost and with the same quality as the standard substrates usually employed. In the case of silicon, for example, note the availability of 200 and 300 mm diameter silicon wafers of standard resistivity, whereas for resistivities in excess of 1 kohm·cm, what is available with a 200 mm diameter is highly unsuitable and there is nothing available at all with a 300 mm diameter. One solution is to process the microcomponents on standard substrates and then, during the final steps, to transfer a thin layer containing the microcomponents onto an insulative substrate such as glass, quartz or sapphire.
From a technical point of view, the major benefit of these transfer operations is that they decorrelate the properties of the layer in which the microcomponents are formed from those of the layer providing the final support, as a result of which these operations are beneficial in many other cases.
The cases may further be cited in which the substrate which is advantageous for processing the microcomponents is excessively costly. In the case of silicon carbide, for example, which offers the best performance (higher temperatures of use, significantly improved maximum powers and frequencies of use, and so on), but whose cost is very high compared to silicon, it would be beneficial to transfer a thin layer from the high-cost substrate (here silicon carbide) onto the low-cost substrate (here silicon) and recover the residue of the high-cost substrate for reuse, possibly after recycling. The transfer operation can take place before, during or after the processing of the microcomponents.
These techniques can also be beneficial in all fields in which obtaining a thin substrate is important for the final application. Power applications may be cited in particular, for reasons associated with the evacuation of heat (which improves as the thickness of the substrate is reduced), or because the electrical current must sometimes pass through the thickness of the substrates, incurring losses that, to a first approximation, are proportional to the thickness through which this current passes. Microchip card applications may also be cited, in which thin substrates are required for reasons of flexibility. Similarly, applications intended for processing 3D circuits and stacked structures may also be cited.
For all these applications (which are cited by way of example), circuit processing is carried out on thick or standard thickness substrates, with the advantages, firstly, of good mechanical resistance to the various technology steps and, secondly, of conforming to the standards relating to their processing on some kinds of production equipment. It is therefore necessary to carry out a thinning operation prior to the final application.
Various techniques can be used to transfer layers from one support to another support. The techniques described in 1985 by T. Hamaguchi et al., Proc. IEDM 1985, p. 688 may be cited, for example. Although these techniques are of great benefit because they transfer a layer from one substrate to another substrate, they necessarily consume the basic substrate (which is destroyed during the course of the process), and cannot transfer a thin film homogeneously unless a stop layer is present, i.e. a layer forming an inhomogeneity in the material of the substrate.
Of the prior art transfer methods that recover the thin film after transfer, it is also possible to use methods of transferring thin layers of materials containing (or not) all or part of a microcomponent. Some of these methods are based on creating a weak buried layer in a material by introducing one or more gaseous species. In this connection, see the following U.S., European, and French patents: U.S. Pat. No. 5,374,564, EP-A-533551, U.S. Pat. No. 6,020,252, EP-A-807970 FR-A-2767416, EP-A-1010198 FR-A-2748850, EP-A-902843 FR-A-2773261, EP-A-963598 which describe these methods. They are generally used with the objective of detaching the whole of a film from an original substrate in order to transfer it onto a support. The thin film obtained can then contain a portion of the original substrate. These films can be used as active layers for processing electronic or optical microcomponents.
In particular, these methods enable the substrate to be reused after separation, as only very little of these substrates is consumed during each cycle. The thickness removed is frequently only a few microns, while the substrates are typically several hundred microns thick. It is thus possible to obtain, in particular, in the method disclosed in U.S. Pat. No. 6,020,252, and European Patent No. EP-A-807970 substrates that are similar to substrates that are “demountable” (i.e. detachable substrates) with the aid of a mechanical stress. This particular method is based on implantation to form a buried weak area in which the cut is made during the final transfer.
Other methods, based on the lift-off principle, also separate a thin layer from the remainder of its original support, again without necessarily consuming the latter. These methods generally employ chemical etching to selectively etch an intermediate buried layer, possibly associated with mechanical forces. This type of method is widely used to transfer III-V elements onto different types of support (cf. C. Camperi et al., IEEE Transaction and Photonics Technology, vol. 3 No. 12, 1991, page 1123). As explained in a paper by P. Demeester et al., Semicond. Sci. Technol. 8, 1993, pages 1124 to 1135, the transfer, which generally takes place after an epitaxial step, can take place before or after processing the microcomponents (it is known as preprocessing or postprocessing, respectively).
Of the methods using a (pre-existing) buried layer that is mechanically weaker than the remainder of the substrate to obtain localized separation at this buried layer, the ELTRAN® process may be cited (Japanese Patent Publication Number 07302889). In this case, a stack based on monocrystalline silicon is locally weakened by forming an area of porous silicon. Another similar case exploits the presence of a buried oxide in the case of a silicon on insulator (SOI) structure, however conventional the latter may be (i.e. even if processed without seeking to achieve any particular demountable effect). If this structure is stuck sufficiently strongly to another substrate and a high stress is applied to the structure, localized fracture can be obtained, preferentially in the oxide, leading to cutting at the scale of the entire substrate. An example of this is described in PHILIPS Journal of Research, Vol. 49 No. 1/2, 1995, pages 53 to 55. Unfortunately, this fracture is difficult to control and necessitates high mechanical stresses, which is not without risk of breaking the substrates or damaging the microcomponents.
The advantage of such weak buried layer methods is the ability to process layers based on crystalline materials (Si, SiC, InP, AsGa, LiNbO3, LiTaO3, etc.) in a range of thicknesses from a few tens of angstrom units (A) to several micrometers (μm), with very good homogeneity. Greater thicknesses are also accessible.
The techniques of transferring layers (with or without microcomponents) based on producing demountable substrates by forming an intermediate layer or weakened interface (whether the weakened interface is obtained by implanting species, forming a porous area, or any other means) run up against problems relating to unintentional delamination if processing prior to the intentional separation is too aggressive.
According to French Patent No. FR 2748851, a demountable substrate of the above kind can be produced by introducing a gaseous species (for example hydrogen). The implanted dose must be chosen so that thermal annealing does not induce any deformation or exfoliation of the surface. Depending on the mechanical forces that can be applied and/or the tool used to induce the separation between the surface layer and the remainder of the substrate, the weakening achieved in the implanted area may prove insufficient. It may then be beneficial to weaken further the implanted area.
The method described in French Patent No. FR 2773261 also produces a demountable substrate, thanks to the presence of a buried layer of inclusions used as a trapping layer in the substrate. After various treatments, for example processing of electronic microcomponents, this localizes, preferably in this trapping layer, sufficient quantities of gaseous species that can contribute to the final separation of the thin surface layer delimited by the area of inclusions and the surface of the substrate. This separation step can include heat treatment and/or the application of mechanical stress to the structure.
The use of this technique can encounter limitations, in particular with regard to the reintroduction of gaseous species into the surface layer after the fabrication of some or all of the microcomponents, which may be undesirable for processing some types of microcomponents.
In the method disclosed in French application No. 0006909, the introduction of a controlled dose of implanted species firstly weakens the buried area (or even overweakens it) and secondly evacuates the gas afterwards, in order to limit a pressure effect in the event of a rise in temperature. There is therefore no deformation or exfoliation of the surface during high-temperature technology steps. This technique necessitates rigorous control of the implantation conditions (dose, temperature, etc.). It may therefore prove beneficial to be able to relax the constraints relating to a narrow technology window.
French Patent No. FR 2758907 proposes local introduction of gaseous species after processing the microcomponents in the surface layer of the substrate. The introduction of these species leads to the formation of a discontinuous buried layer of microcavities that can contribute to the fracture after fastening the treated substrate to a support substrate. Thus the substrate is weakened after processing the various microcomponent fabrication technology steps. The accessible size of the areas to be masked (which correspond to the active areas of the microcomponents) may prove to be a limitation, depending on the intended applications. For example, this technique is difficult to use for microcomponent sizes from a few tens of microns to a few hundred microns. Moreover, depending on the microcomponent production technology used, the thickness of the active layer, i.e. the layer incorporating the microcomponents, can be as much as several microns. The introduction of gaseous species at a great depth (by dedicated implanters or by accelerators), with adequate protection of the areas by masking, can then prove difficult.